In the manufacture and testing of medical devices, mechanisms are used to radially compress cylindrical devices such as stents, balloons, and catheters. For example, installation of a stent onto a catheter balloon is typically accomplished by compressing the stent radially inward onto the balloon with enough pressure to permanently deform the stent to a smaller diameter and to slightly embed the metal stent into the plastic balloon. In another example, a polymer catheter balloon is compressed radially after pleating to wrap it tightly around the catheter shaft. In another example, a self-expanding stent is radially compressed to insert it into a sheath or delivery system. In an example of medical device testing, a stent is radially compressed while the required force is measured, in order to measure the stent's functional relationship between diameter and radial force.
A first type of prior art device includes a radial compression mechanism wherein several similar wedge-shaped die with planar surfaces are arranged to form an approximately cylindrical central cavity, the wedges being hinged and driven in unison to change the diameter of the cavity. Examples of this mechanism are the Crimpfox tool sold by Phoenix Contact GmbH 7 Co. KG (CRIMPFOX UD 6-6, Part Number 1206366), and the “segmental compression mechanism” marketed by Machine Solutions Incorporated, and described in U.S. Pat. No. 6,968,607. In this type of mechanism, the working surfaces of the die have a wedge shape with two planar surfaces meeting at the tip. A shortcoming of this type of mechanism is that there exists a gap between adjacent wedges, the size of which varies with the diameter of the cavity in an undesirable way. Typically, the mechanism is specifically designed to provide a desired range of cavity diameters. At the lowest and highest diameters, the die are tightly wedged against each other (zero gap). As the diameter is increased from the lowest, the gap increases until it reaches a maximum, then decreases until it becomes zero again at the highest diameter. The diameter range and gap (as a function of diameter) depend on the specific design of the mechanism, particularly the location of the hinge point of the die and the diameter of the circle formed by all of the die hinge points in the mechanism. A larger diameter of the hinge point circle results in a smaller maximum gap for a given diameter range. The strict design tradeoffs for this type of mechanism results in a mechanism that must be large to provide a small maximum gap for a given diameter range, or a mechanism that must have a large gap to provide the same diameter range in a small size. Large gaps between the wedges are a disadvantage because they allow space for parts of the compressed device to go into. For example, the metal struts of a stent can move into the gap and be damaged.
A second type of prior art device includes a radial compression mechanism wherein several similar wedge-shaped die with planar surfaces are arranged to form an approximately cylindrical central cavity, the wedges being attached to linear guides that constrain each die individually to move linearly relative to a stationary part, the die being driven in unison to change the diameter of the central cavity. Each die's motion path is constrained when assembled to the stationary part, even when the other die are not present. The die are guided only by the linear guides, and not by neighboring die. Examples of this mechanism include the mechanism taught by Kokish in U.S. Pat. No. 6,651,478, or the mechanism marketed by Interface Catheter Solutions as part of the model DFW-1000 balloon fluter-wrapper machine. In this type of mechanism, the working surfaces of the die have a wedge shape with two planar surfaces meeting at the tip. The linear motion of the wedges in this mechanism provides a wedge-to-wedge gap that is constant, independent of the cavity diameter, and may be designed to be any desired size. A shortcoming of this mechanism is that it typically does not provide a sufficiently accurate positional relationship of the wedge-shaped working ends of the die. Accurate positional relationship of the die is important so that the central cavity remains approximately round and provides even compression around the circumference of the compressed device, and so that the largest die-to-die gaps aren't much larger than the average. Each die is carried on its own linear guide, all of the guides are attached to a plate or base, and another rotating part such as a cam must be used to impose motion in unison. Therefore, many parts and attachments may influence the accuracy (roundness) of the central cavity. Medical device manufacturing and testing often require an accurately round cavity at diameters as small as 0.3 mm, which this type of mechanism is typically unable to achieve because of dimensional variability of the many parts.
A third type of prior art device is a radial compression mechanism wherein several arcuate-shaped die have an outer end pivotally attached to a hinge plate and approximately wedge-shaped inner working tips that form an approximately cylindrical-shaped central cavity. The die are driven in unison to form and change the diameter of the central cavity. A mechanism of this type is described in U.S. Pat. No. 7,963,142 B2, and is marketed by Blockwise Engineering LLC as the “J-Crimp” mechanism. This type of mechanism has an important advantage over the first type of prior art: constant die-to-die gaps that do not vary with opening diameter. It also has an important advantage over the second type of prior art: it can be manufactured with die-to-die gaps smaller and more precise. Although the die-to-die gaps of this third type of mechanism can be made not to vary as function of the opening diameter, and are generally smaller than those of the other prior art, the tolerance of the die-to-die gaps remains significant because there are several parts contributing to manufacturing dimensional variability, including the die themselves, which are rather long and thin. Further improvement of the smallness and precision of the die-to-die gaps would be advantageous in some applications, such as stent crimping of very small stents or balloon wrapping of very small balloons, where the compressed product benefits from being very small, uniform, and accurately round.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved radial compression mechanism.
Another object of the invention is to provide a new and improved radial compression mechanism for compressing devices such as stents, catheters, balloons, and the like in the medical industry.